<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <p id="p">hello world</p>
    <button id="btn">btn</button>
    <script>
        var btn=document.getElementById("btn");
        btn.onclick=function(){
            var self=this;
            setTimeout(function(){
                console.log(self.id)
            },1000)
        }
    </script>
</body>
</html>